I plugged the numbers into BikeGeo too. Fork length was not listed for Cervelo and Offset was not listed for the Orbea. I adjusted those numbers (386 and 52) until the tool gave me almost the exact mfg. listed stack, reach and wheelbase for the R5 and was 1 mm off on the stack/reach for the Terra.
When I did that. the stem reach was 473 and 471 and stack was 586 and 588 - which are essentially rounding errors. So, yeah, your math should be right and the fit should be the same for the two bikes.
Are you sure you have a -6 and -12 stems?